The present invention relates to a magnetic drive field coil arrangement for producing a rotating magnetic field to induce propagation of cylindrical domains in bubble memories disposed therein and, more specifically, to a coil arrangement having access openings to the central region of the coil arrangement.
The storage of digital data by means of cylindrical magnetic domains in thin, single crystal films of certain orthoferrite or garnet materials has been known for some time. Such so called bubble memories can utilize domain propagation via localized fields from current pulsed conductors in contact with the film or more commonly from an etched pattern in a magnetic overlay pattern excited by an applied rotating magnetic field. This rotating magnetic field consists of two mutually perpendicular components, both in the plane of the magnetic film domain material. They are sinusoidal in their time variations and are nominally 90.degree. apart in time phase. The coils used to produce this rotating field must be designed to accommodate the physical dimensions of the bubble memory and to produce a sufficiently uniform field over the required region. Also, their electrical properties must be such that sufficient currents can be maintained at the desired frequency without excessive voltage or heating problems. Thus, the inductance and resistance of the coils must be maintained as low as possible. It is also most desirable that the coils be constructed so there is easy access to the useable field volume to facilitate inserting and removing the bubble domain memory and its associate lead wiring without disassembling the coils. The coils should be sufficiently open to allow circulation of a coolant, such as air, to the magnetic film and also be designed such as air, to the magnetic film and also be desinged such that they can be wound or otherwise formed readily.
Bubble memory devices are usually constructed from a non-ferrimagnetic crystalline wafer substrate which is about one millimeter thick and which is covered on one side with an epitaxially grown ferrimagnetic ferrite film. The permalloy pattern used to accomplish domain propagation and any conductor patterns required for domain control or detection are either deposited on top of the magnetic film or on a separate glass plate pressed in contact with the magnetic film. In either case, the assembly is about one millimeter thick, but it can cover an area of a few square centimeters. This area can be occupied by one wafer or an assembly of several wafers side by side. They are nominally mounted in some manner on a flat support board composed of ceramic or fiberglass-epoxy composite. Leads to carry electrical signals to and from the wafers are usually formed by photoetching leads on this support board. The required magnetic fields must therefore be produced uniformly throughout a volume which is high enough to clear the relatively flat support board and its attached wafers and of sufficient area to include the entire surface area of the bubble memory devices. In addition to providing such uniform fields, the coil geometry should allow exit of the required leads and provide easy access for the insertion of the memory wafers and the flow of a coolant medium. Flat rectangular solenoids which fit together with their axes perpendicular are known, but such solenoid arrangements usually fail to satisfy one or more of the desirable requirements outlined above. More specifically, the solenoid arrangement does not have open sides. Thus, the arrangement must be disassembled to gain access to the bubble memories disposed therein. Additionally, a cooling medium can not readily flow past the memory devices while they are in use within the closed coil arrangement.